1. Field of the Invention.
This invention relates to a method of, and apparatus for, melting and casting metal. The term "metal" is used herein to include metal alloys.
2. Description of the Prior Art.
A widely used known method of making metal castings comprises the following main steps:
(i) melting is carried out in a melting vessel such as a furnace or large crucible which is then tilted to pour the metal; PA1 (ii) into a smaller transfer crucible or launder in which the metal is transferred to a casting station at which there is a mould, and PA1 (iii) casting is carried out by pouring the metal from the transfer crucible or launder into the mould. PA1 (a) lower casting temperatures, reducing hydrogen pick-up, oxidation and metal losses, and raising productivity by increasing freezing rate of the casting in the mould; PA1 (b) increased fluidity, enabling thinner sections to be cast over larger areas, without recourse to very high casting temperatures; PA1 (c) because of the `skin-freezing` characteristics of solidification of eutectic alloys (as contrasted with pasty freezing of long freezing range alloys), any porosity is not usually linked to the surface and so castings are leak-tight and pressure-tight. This is vital for many automobile and hydraulic components. The concentrated porosity which might be present in the centre of an unfed or poorly fed section can be viewed as usually relatively harmless, or can in any case be relatively easily removed by the foundryman. The castings in such alloys tend therefore to be relatively free from major defects.
Sometimes a modified known method is used in which the metal is poured directly from the furnace into the mould, eliminating the transfer stage (i.e. stage (ii) above.
Less frequently, another modified known method is used in which after melting and pouring into a transfer ladle, metal is poured into a furnace or crucible contained within a pressure vessel. The pressure vessel is sealed and then pressurised by a gas which displaces the liquid metal up a riser tube and into the mould. This method of casting is called low pressure casting. It has the commendable feature that the pouring into the casting is replaced by an upward displacement which is much less turbulent than pouring under gravity. Correspondingly higher quality castings are produced than are produced with pouring under gravity. However, optimum quality is not attainable in oxide-forming metals, such as those containing relatively large quantities of aluminum and magnesium, since surface oxides are entrained within the metal by the turbulence involved in the previous transfers carried out by pouring, and the entrained oxides do not separate quickly from the liquid.
Most of the above described methods result in a total free fall of metal under gravity in one or two steps, occasionally more, through a vertical distance of from 0.50 metres to several metres. The resulting high metal velocities give rise to severe splashing and churning.
In a rarely used known method, the metal is melted in a crucible or furnace connected directly to a mould, the crucible or furnace is then pressurised, or the mould subjected to partial evacuation, so that metal is forced or drawn up into the mould cavity directly. This method of casting eliminates all turbulence from transfers in casting and is therefore capable of making high quality castings in oxidisable alloys. Unfortunately, however, the method by its nature is limited to batch production. Also any treatment of the metal, such as de-gassing by bubbling gases through the liquid, or fluxing by stirring in fluxes, involves the danger of residual foreign material suspended in the liquid metal. There is no intermediate stage in which such defects can conveniently be filtered out. The time usually allowed in consequence in an attempt to allow such impurities to sink or float prior to casting involves a considerable time delay and thus represents a serious reduction in the productivity of the plant.
All of these known methods therefore suffer from the problem of not providing high productivity together with high quality of castings.
An attempt to provide a solution to the above problem is described in Engineering, Vol. 221, No. 3, Mar. 1981, LONDON (GB) J. Campbell "Production of high technology aluminium alloy castings" Pages 185-188.
This discloses a method of melting and casting metal comprising the steps of melting metal in a melting vessel, transferring metal from the melting vessel into a casting vessel by flow of metal under gravity and pumping metal against gravity from the casting vessel into a mould. However, whilst some improvement over previously known methods was experienced, as high productivity with high quality of casting as was desired was not achieved.